Conventional image forming devices such as a laser printer use a photosensitive cartridge in which a photosensitive drum is rotatably supported. A developer cartridge is also used therein for supplying the photosensitive drum with toner. The photosensitive cartridge and the developer cartridge are mountable to and detachable from the main body of the device. Only necessary one or ones of the cartridges can be replaced with a new one when the lifetime of the cartridge is ended.
Japanese Patent Application Publication No. 2000-267547 discloses a photosensitive cartridge, for use in an image forming device. The photosensitive cartridge has a flat lower surface facilitating to place the cartridge removed from the image forming device on a flat table.
With the photosensitive cartridge according to the proposal described above, the cartridge can be placed stably on a flat plane. However, the shape of the upper surface of the cartridge does not allow another photosensitive cartridge to be stacked thereon. Typically, a manufacturer collects and stores, in a stock room, old photosensitive cartridges or developer cartridges detached from the main body of the device for recycling In this case, it is convenient if the cartridges can be stacked one on the other. Otherwise, inconvenience is caused in handling the old cartridges and a large space is required for storage.